In a manner well known to persons familiar with this technology, diesel engines efficiently convert the latent heat of hydrocarbon fuel into useful mechanical power. In operation, a metered amount of fuel is injected into each cylinder of the engine at recurrent intervals synchronized with rotation of the engine crankshaft to coincide with the air-compression stroke of a reciprocating piston. As pressure increases, the compression temperature in the cylinder rises and the injected fuel is soon hot enough to ignite. The resulting combustion or firing of fuel in the cylinder forces the piston to move in the opposite direction, thereby applying torque to the engine crankshaft.
The conventional engine fuel is a relatively low grade, refined petroleum known generally as diesel fuel oil which has desirable ignition and heat release characteristics. Diesel fuel oil has acceptably low levels of corrosive, abrasive and other noxious matter, and it is in ample supply at the present time. But for nearly a century persons skilled in this art have known that coal, in the form of either a dry powder or a liquid slurry (i.e., a mixture of pulverized coal or other form of carbon dust and a liquid carrier such as oil or water), is an alternative fuel for diesel engines. Interest in developing a practical coal-fueled diesel engine has varied over the years directly with the cost and inversely with the supply of standard diesel fuel oil. For a review of such development efforts, see the article entitled "Slow-Speed Two-Stroke Diesel Engine Tests Using Coal-Based Fuels" by J. P. Davis, J. B. Dunlay, M. K. Eberle, and H. A. Steiger, published in 1981 as paper No. 81-DGP-12 by the American Society of Mechanical Engineers (New York, N.Y., U.S.A.).
The injection of a mixture of coal and water (hereinafter sometimes referred to as "CWM") into a compression ignition reciprocating internal combustion engine such as a large, medium-speed, multicylinder diesel engine, poses problems not typically encountered in the injection of pure liquid fuels. In such a mixture there are fine particles of coal which can cause excessive rates of wear and premature failure of components in a fuel injection system. CWM fuel includes, for example, from less than 30% (by weight) to as much as 60% coal particles in the range of from 0.1 to 50.0 microns in diameter. These particles are both abrasive and corrosive to the materials used in the fuel injection system. Furthermore, the ignition delay time of CWM is undersirably long, typically, five to six times longer than the ignition delay time of standard diesel fuel oil.
A fuel injection system capable of reliably injecting CWM fuels must not only be capable of avoiding the premature wear problems described above, but must also be capable of injecting the fuel in a manner to obtain proper combustion in each cylinder of the diesel engine. To obtain proper combustion the CWM has to be discharged into the combustion chamber in a finely atomized state. Very high injection pressures (e.g., on the order of 10,000 to 15,000 psi) are required to cause sufficient atomization of the CWM so that this fuel will mix adequately with air in the combustion chamber for ignition and complete burnout in the time available (e.g., 10 to 12 milliseconds in an engine running at 1,000 rpm), and such high injection presses need to be sustained to ensure good combustion at reduced loads/speed. Further, the CWM fuel injection must be closely metered in quantity and time.
In a conventional positive displacement injector assembly, a spring-loaded needle valve moves reciprocally inside a hollow nozzle, guided in the cylindrical opening or bore of the injector body. A very close, smooth fit is maintained between the cylindrical surface of the needle and the guiding surface of the injector body, and a small amount of liquid fuel is allowed to leak past the needle to provide lubrication and ensure freedom of movement. Normally the needle is held down by spring pressure in an orifice-blocking position, in which a conical surface at its lower end sealingly mates with a correspondingly shaped valve seat in the vicinity of the nozzle tip. Periodically a pressurized charge of diesel fuel oil is delivered to an inlet port of the injector by an external fuel pump. This charge exerts hydraulic pressure on the conical surface of the needle to compress the hold-down spring and lift the needle a short distance, thereby opening the orifice in the valve seat and allowing the pressurized diesel fuel to spray into the associated cylinder via one or more holes in the nozzle tip. For stable operation, the pressure supplied by the associated fuel pump will continue to rise after the needle lifts and throughout most of the injection interval. The needle will return to its orifice-closing position as soon as the fuel injection pressure drops below the spring pressure.
While a positive displacement type of injector system enables the amount and timing of diesel fuel injection to be precisely metered, the injection pressure in a practical injector of this type is limited to a relatively low magnitude of from 3,000 to 4,000 psi at the initiation of needle lift, and its maximum magnitude will decrease as engine load and speed are reduced. In order to obtain efficient and complete combustion in a coal-fueled engine, much higher injection pressure will be needed for adequate atomization of the more viscous CWM fuel. Good atomization is vitally important in the early, pre-ignition phase of each injection interval when the compression temperature is relatively low and rising. Ignition and combustion will both be helped, at both full load and part load, by injecting the CWM fuel at a pressure that is initially high enough to ensure good atomization and that is not reduced with engine load. For these reasons, a high-pressure accumulator type of injector assembly is better suited for injecting CWM fuel. In such an assembly, diesel fuel is accumulated and stored under the desired high pressure in a chamber that communicates with the space around the needle valve adjacent to the valve seat. The needle is maintained in its closed position, against the pressure being exerted on its conical end by the fuel, by an externally controlled hydraulic system or the like. Injection is permitted when the hold-down pressure of the hydraulic system is released or when an overriding hydraulic pressure is applied to the needle valve in the needle-lifting direction. See, for example, British Patent No. 494,951 and U.S. Pat. No. 1,843,410.
In an injector for CWM fuel, the particulate matter in the fuel will tend to migrate into the clearance gap between the needle valve and its guiding barrel, thereby abrading the bearing surfaces and/or causing seizure of the needle valve in the injector assembly. Because of this problem, an injector used with coal slurry fuels is likely to experience clogging and to have an undesirably short and unreliable service life. In order to increase the useful life of a CWM injector, it has heretofore been suggested to increase slightly the normally small, uniform clearance gap around the guided portion of the needle valve and to introduce compressed air, clean fuel, or lube oil under high pressure in back of the valve so as to flush out or purge the gap and inhibit ingress of the undesirable particles. But poor guidance, valve seating problems, and more wear are likely to result as the clearance gap increases.
More than 65 years ago it was recognized as desirable to inject a small amount of readily ignitable pilot fuel in diesel engines to improve combustion of "heavy" hydrocarbon fuels that are otherwise difficult to ignite. See British Patent No. 124,642. As used herein, the term "pilot fuel" means relatively light hydrocarbon fuel (e.g. methanol or even standard diesel fuel oil) characterized by being significantly easier to ignite than the primary fuel in the injection system. In a practical coal-fueled diesel engine, the injection of a small quantity of pure diesel fuel to aid ignition of the CWM fuel is particularly advantageous because CWM has a relatively long ignition delay time, because there are practical limits to the degree of atomization of CWM that can be obtained, and because there are practical limits in the amount that the inlet air temperature and the compression temperature of the engine cylinders can be increased compared to diesel engines using standard diesel fuel oil as their primary fuel. Obviously the pilot fuel can be introduced by mixing it with the CWM in the fuel supply tank. Alternatively, a separate pilot fuel injector can be used (U.S. Pat. No. 4,335,684), or the pilot and main injectors can be combined in one coaxial assembly (see U.S. Pat. No. 4,266,727). In any event, fuel costs will be saved (assuming that CWM fuel is less expensive than pilot fuel) by injecting the smallest amount of pilot fuel consistent with timely ignition of the CWM fuel.